1. Field of the Invention
The present invention relates to light diffusive coatings which are formed on the base surface of lamps such as halogen lamps and high pressure discharge lamp bulbs, and optical fibers and window glass, etc., and to methods of forming such coatings.
2. Discussion of the Background
To obtain a uniform distribution of illumination over the illuminated surface, halogen lamps for photocopiers, for example, have several filaments connected in series through conductors, and the whole assembly is enclosed in a straight tube type outer vessel made of silica glass so that it lies along the axis of the tube. However, because this halogen lamp includes a large amount of infrared radiation in its emissions, there is a danger that the item being photocopied will be damaged by the heat. For this reason, the lamp surface generally undergoes a honing treatment to provide a diffusive surface.
This method, however, has the disadvantage of being very laborious. In addition, the methods of forming a light diffusive coating by applying a fine diffusive powder such as silica to the outside of the lamp by electrostatic coating and etching may be considered. These types of coating are generally weak for mechanical stresses, wear easily, and in some cases the light diffusive effect falls off over long periods of use. Depending on the type of fine light diffusive power, there are cases in which the coating has a poor affinity for the quartz glass, and readily peels off.
With regard to these points, the inventors have previously developed the technology of forming a coating on the outer surface of the transparent outer vessel which allows visible light to pass through, but reflects infrared light, and by providing a light diffusive coating on top of the infrared reflective coating, the visible light which has passed through the infrared reflective coating is dispersed by the light diffusive coating, giving a uniform distribution of illumination on the illuminated surface. This was proposed in the Japanese Patent Application Number Sho 58-95001(95001/1983).
However, when this technology is applied to lamps, such as halogen lamps for example, because the temperature of the lamp's outer vessel is extremely high, with some light diffusive coatings there is the danger that the light diffusive coating may peel off due to the effect of repeated switching on and off over a long period of time. This means that it is essential to select a light diffusive coating which is stable regardless of the rise and fall in temperature and which is mechanically strong.
A light diffusive coating which includes bubbles and pits within a layer of metal oxide and a method for forming such a coating is disclosed in Japanese Patent Applications P59-203672 and P59-203673 and U.S. Pat. No. 4,721,877 and U.S. patent application Ser. No. 07/098,884. Although the disclosed coating provides a high diffusiveness, the presence of the bubbles decreases the light transmission and mechanical strength of the coating.
Thus, there remains a need for a light diffusive coating with improved light transmission and mechanical strength while maintaining adequate light diffusion. In addition there is a need for a method of production of such a light diffusive coating.